In physics class, the girl to boy ratio is $6$ to $3$. If there are $15$ boys, how many girls are there?
Explanation: A ratio of $6$ girls to $3$ boys means that there is a group of $6$ girls for every group of $3$ boys. If there are $15$ boys, then there are $5$ groups of $3$ boys each. Because we know the ratio is $6:3$ , there must be $5$ groups of $6$ girls each. There is a total of $30$ girls in physics class.